[Technical Field]
The present invention relates to a nuclear power plant and more particularly to a nuclear power plant applicable to a boiling water nuclear power plant having comparatively low thermal power.
[Background Art]
In the nuclear power plant (for example, the boiling water nuclear power plant), even after the operation stop, it is necessary to supply cooling water to a core in a reactor pressure vessel and cool a plurality of fuel assemblies loaded in the core to remove decay heat generated in a nuclear fuel material included in the fuel assemblies. Generally, after the operation stop of the nuclear power plant, a part of the cooling water in the reactor pressure vessel is discharged into a pipe connected to the reactor pressure vessel, and the discharged cooling water is cooled by heat-exchanging it with seawater in a heat exchanger connected to the pipe, and is returned to the reactor pressure vessel through a return pipe of the cooling water that is cooled. As mentioned above, after the operation stop of the nuclear power plant, the cooling water in the reactor pressure vessel is heat-exchanged with seawater, thus the decay heat of the nuclear fuel material is removed.
Such a nuclear power plant uses a motor-driven pump to supply the cooling water in the reactor pressure vessel to the heat exchanger and supply seawater to the heat exchanger, and electricity for driving the motor-driven pump is necessary to remove the decay heat after the stop of the nuclear power plant. When an abnormal event of external power loss occurs at the time of stop of the nuclear power plant, an emergency generator is driven, and the motor-driven pump is driven, and the decay heat when the nuclear power plant is not in operation is removed.
A reactor cooling system where, when a loss-of-coolant accident occurs, the safety of the core can be ensured by forces of nature without using a dynamic device and both removal of the decay heat in the reactor and a water injection function are achieved by a same facility, is proposed in Japanese Patent Laid-Open No. 62(1987)-182697. In this reactor cooling system, a tank including a body filled with water disposed at a higher position than the reactor pressure vessel and a pipe passing through longitudinally in the body is disposed at a higher position than the reactor pressure vessel. At the time of a loss-of-coolant accident, the steam in the reactor pressure vessel is discharged and condensed in the water in the body and the water in the body is injected into the reactor pressure vessel. At the time of an anticipated operational occurrence that a main condenser cannot be used due to a turbine trip, the steam in the reactor pressure vessel is introduced into the pipe and is cooled by the water in the body, and the steam is condensed in the pipe by this cooling and the generated condensed water is injected into the reactor pressure vessel. In the reactor cooling system described in Japanese Patent Laid-Open No. 62(1987)-182697, it is possible to cool the steam in the reactor pressure vessel and inject the condensed water into the reactor pressure vessel by gravity both in a loss-of-coolant accident and at an anticipated operational occurrence.
Japanese Patent Laid-open No. 2011-58866 describes a nuclear power plant having a reactor isolation condenser for cooling the fuel assemblies in the core when a station blackout occurs and the reactor enters an isolation state, and a gravity-driven cooling system. The reactor isolation condenser is provided with a condenser pool disposed above the reactor pressure vessel for storing cooling water, a condenser heat exchanger installed in the cooling water in the condenser pool, a steam supply pipe connected into a steam space in the reactor pressure vessel and connected to the condenser heat exchanger, and a condensed-water return pipe connected to the condenser heat exchanger, and the reactor pressure vessel. The gravity-driven cooling system is provided with a gravity-driven cooling system pool which is disposed above the core in the reactor pressure vessel and is filled with cooling water, and an injection pipe for connecting the gravity-driven cooling system pool and the reactor pressure vessel.
When a station blackout occurred and the reactor entered an isolation state, the steam in the reactor pressure vessel is introduced to the condenser heat exchanger through the steam supply pipe and is condensed by the cooling water in the condenser pool. The condensed water generated by the condensation is returned to the reactor pressure vessel through the condensed-water return pipe. The cooling of the core in the reactor pressure vessel is enabled by the reactor isolation condenser even if a station blackout occurs and the reactor enters an isolation state. Further, when a loss-of-coolant accident occurs, the cooling water in the gravity-driven cooling system pool is supply to the core through the injection pipe.